Some pointers to note:
Small patterns in soft, light and muted colors are easy to mix and match.
Large patterns in bright colors have bigger impact.

So, how do you mix and match patterns?

Compare Tiffany Table Lamp Pattern
The Floral Collection, Basket Style, Porcelain

Look For Bargain Patterned Area Rug
Indian Storm Pattern, Brown Circle, Contemporary, Oriental

Use Same Color Palette
As a rule of thumb, patterns work well together if they share the same color palette. For example, if you have a striped blue and white couch in your living room, you can decorate it with throw pillows with a flower motif or any other patterns of the same color as the couch.

Mix With Plains
Another safe way is to combine the use of plains and patterns together. In order to achieve color harmony when mixing plains and patterns, opt for monochromatic color scheme.

Repeat Pattern
You could also repeat the pattern, or variations of it over the entire room, to get a collective look. For example, if you're using a spotted curtains, you can repeat the spot or variation of it on lampshades, rugs or even furnitures, even if the repeated spots are not of the same color. Mixing colors of the same family however, works all the time.

Be Adventurous
If you're adventurous, experiment mixing various kinds of patterns of different colors. For example, the oriental rugs have intrinsic character which makes it possible to use several in one room despite their variation in designs, patterns and colors.

Use A Common Theme
Over time, when you have developed an eye for detail, you might one to try mix and match patterns of the same theme. For example, you can mix together a collection of vintage fabrics, rugs, furnitures or art deco from the seventies, eventhough they do not share the same colors and patterns.